Pain Behind the Knee: Causes, Baker’s Cyst and When to Seek Care

Short answer: Pain behind the knee can come from a Baker’s cyst, swelling inside the joint, osteoarthritis, a meniscal problem, or the hamstring, calf and other soft tissues crossing the back of the knee. Less commonly, posterior pain and swelling may involve a blood clot, blood vessel, infection or another condition requiring urgent care. The exact location, presence of a lump, calf swelling, injury and movement pattern guide the assessment.

The back of the knee—called the popliteal region—contains more than the knee joint. Tendons, muscles, nerves, arteries and veins pass through a relatively small space. This is why a scan selected only because “the pain is behind the knee” may miss the real clinical question. Examination should first determine whether the pattern appears joint-related, muscular, cystic, neurological or vascular.

When pain behind the knee needs urgent assessment

Seek urgent medical assessment if:

• the calf or whole leg suddenly becomes swollen, painful, warm or discoloured;
• posterior knee or calf pain develops after surgery, prolonged immobility or a long journey, or with a previous blood clot or active cancer;
• the knee is hot, very swollen and intensely painful, particularly with fever or feeling seriously unwell;
• there was major trauma, rapid swelling, deformity, inability to bear weight or true locking;
• the foot becomes cold, pale, blue, numb or markedly weak;
• a lump is pulsating, rapidly enlarging or associated with circulation symptoms.

Sudden breathlessness, chest pain, fainting or coughing blood is an emergency because a clot in the lungs is one possible cause. In Singapore, call SCDF 995 for a life-threatening emergency.^[1]

What does the pain feel like?

Different descriptions can provide useful—not diagnostic—clues:

• Fullness or pressure: may occur with a joint effusion or Baker’s cyst.
• A localised lump: may represent a cyst or another soft-tissue or vascular structure and should be examined if new.
• Sharp pain with deep bending: can arise from compressed posterior structures, meniscus or joint swelling.
• Pulling pain when straightening: may involve tight or irritated posterior tissues.
• Pain after running: may reflect a calf, hamstring, popliteus or joint-load problem.
• Burning, tingling or electric pain: can suggest nerve involvement or referred pain.
• Calf ache with one-sided swelling: requires consideration of a venous problem.

Record whether the pain is central, inner-back or outer-back; whether a lump changes with knee position; and whether symptoms extend into the calf or thigh.

1. Baker’s cyst or popliteal cyst

A Baker’s cyst is a fluid-filled swelling at the back of the knee. It often forms when excess joint fluid moves into a communicating sac behind the knee. It can be present without symptoms or cause a sense of fullness, stiffness, tightness, pain and lower-leg swelling.

The American Academy of Orthopaedic Surgeons explains that Baker’s cysts in adults commonly occur alongside an underlying knee problem such as osteoarthritis, rheumatoid arthritis, a meniscal tear or ligament injury.^[2] The cyst is therefore often a sign of activity within the joint rather than an isolated object that should simply be removed.

Symptoms can worsen with prolonged standing, activity, full bending or straightening. A large cyst may limit movement. The size of the lump does not always match pain severity, and some cysts are discovered incidentally on examination or imaging.

Can a Baker’s cyst burst?

A Baker’s cyst can occasionally rupture, allowing fluid to track into the calf. This may cause sudden pain, swelling, warmth or skin colour change. These symptoms can resemble a deep-vein thrombosis (DVT), and patients cannot reliably distinguish the two at home. NHS guidance advises urgent evaluation when leg pain or swelling suddenly worsens because rupture and DVT are among the possibilities.^[3]

Do not deeply massage a newly swollen calf or assume that a known cyst explains every new episode. A clinician may use a structured clot-risk assessment and venous ultrasound where indicated.

2. Knee osteoarthritis and joint effusion

Osteoarthritis can cause activity-related pain, stiffness, reduced movement, crepitus and swelling. Extra fluid inside the joint may create pressure that is felt at the back, even when no distinct cyst is visible.

NICE advises that osteoarthritis can usually be diagnosed clinically without routine imaging in people aged 45 or over who have activity-related joint pain and either no morning stiffness or stiffness lasting no more than 30 minutes, unless atypical features suggest another or additional diagnosis.^[4]

A new hot joint, large unexplained effusion, severe rest pain or systemic illness should not automatically be attributed to osteoarthritis. Crystal arthritis, infection or injury may need to be considered.

3. Meniscal symptoms

The posterior part of the medial or lateral meniscus can contribute to pain felt towards the back of the joint. A meniscal problem may follow twisting on a planted foot, deep loaded bending or a sports injury. Degenerative changes can also become symptomatic without a dramatic event.

Possible associated symptoms include joint-line pain, stiffness, swelling, catching, locking, giving way or reduced movement. AAOS lists these among common symptoms of a meniscal tear.^[5] Posterior pain alone is not enough to diagnose one.

An MRI may show a meniscal abnormality, but the clinician still has to decide whether it fits the patient’s pain pattern. Age-related findings may not be the dominant pain source.

4. Hamstring tendon pain

Hamstring tendons cross the back and sides of the knee. Pain may be felt towards the inner-back or outer-back region after sprinting, hills, sudden acceleration, stretching or strength work. Tenderness may be more localised to a tendon than to the centre of the joint.

A gradual load-related tendon problem differs from an acute tear. Sudden pain with a snapping sensation, bruising, a palpable gap or marked weakness requires assessment. Treatment depends on the location and severity rather than assuming all posterior tightness needs stretching.

5. Calf muscle or upper-calf injury

The gastrocnemius muscle begins above the knee and crosses the joint. A strain can therefore create pain at the back of the knee or upper calf after running, jumping or a sudden push-off. Pain may be reproduced by calf contraction or stretch and accompanied by bruising.

A calf strain and DVT can share symptoms. Injury history makes a strain more plausible but does not automatically rule out a clot, particularly when swelling is substantial or risk factors are present.

6. Popliteus and other posterior soft tissues

The popliteus is a small muscle at the back of the knee that contributes to control during bending, rotation and downhill movement. It can be considered in selected running or twisting presentations, although it is not possible to identify reliably through symptom location alone.

Other posterior structures include the joint capsule and smaller tendons. Clinical assessment may use resisted movements, palpation and functional tasks to decide whether the pain is more likely joint-related or extra-articular—meaning outside the joint.

7. Deep-vein thrombosis

DVT is a clot in a deep vein, usually in the leg. Symptoms may include one-sided calf or thigh pain, swelling, warmth, skin colour change or prominent veins, but not every patient has the same pattern.

Risk can be influenced by recent surgery, hospitalisation, prolonged immobility or travel, previous clots, active cancer, pregnancy and certain hormone-containing medicines. NICE recommends assessment using the medical history, physical examination and a structured clinical-probability pathway, followed by D-dimer testing and/or proximal leg-vein ultrasound as appropriate.^[6]

A knee MRI is not the standard test for a suspected DVT. If clot is a realistic concern, the urgent vascular pathway takes priority.

8. Nerve or referred pain

Pain behind the knee can be referred from the hip, lower back or nerves in the leg. Burning, tingling, numbness, pain extending along the limb, back pain or weakness may prompt a neurological examination.

A normal knee scan does not rule out a referred source. The clinician may need to examine the back, hip, pulses, sensation and muscle power instead of repeatedly treating the posterior knee itself.

9. Less common vascular, infectious or mass-related causes

A popliteal artery aneurysm, infection or tumour is uncommon but can present with posterior swelling or pain. A pulsating mass, circulation change, a new enlarging lump, unexplained progressive rest pain, fever, night sweats, weight loss or previous cancer makes assessment more important.

Most pain behind the knee is not caused by a tumour or arterial problem. These possibilities are included so that atypical findings are not repeatedly labelled as a cyst or muscle strain without examination.

Why does it hurt when bending or straightening?

Deep bending compresses posterior tissues and may increase the pressure from an effusion or cyst. Full straightening tensions some tendons and the posterior capsule. A structure can therefore hurt at one end of movement and not the other.

If the knee is physically blocked rather than merely painful, do not force it. True locking after a twist may involve an internal joint problem and needs timely assessment.

Why does pain appear after walking, running or standing?

Prolonged weight-bearing can make a joint effusion, osteoarthritis flare or cyst more noticeable. Running, hills and acceleration increase demand on the calf, hamstring and smaller posterior structures. The useful question is whether the symptoms are:

• present at the start or only after a predictable duration;
• associated with swelling or a lump;
• worse on hills, speed work or deep bending;
• settled by the next morning or progressively worsening;
• accompanied by calf swelling, breathlessness or another warning feature.

What can you do for mild posterior knee pain?

If there was no major injury, no substantial swelling and no vascular or systemic warning feature, short-term measures may include:

• reduce the activity that clearly provoked symptoms;
• maintain comfortable movement rather than prolonged complete rest;
• avoid repeatedly testing deep bends or forceful stretches;
• use a wrapped cold pack briefly for a recently irritated or swollen area if skin sensation and circulation are normal;
• record changes in the lump, calf size, movement and weight-bearing;
• arrange assessment if pain persists, recurs or progresses.

Do not attempt to drain a lump yourself. Avoid strong massage when the diagnosis is uncertain, particularly with new calf swelling or bruising.

How pain behind the knee is assessed

The history should clarify onset, exact location, injury, activity, swelling, a lump, locking, calf symptoms, recent travel or surgery and clot risk. Previous arthritis, inflammatory disease and meniscal injury may be relevant.

Clinical examination may include:

• comparison of knee, calf and whole-leg swelling;
• inspection and palpation of a posterior lump;
• active and passive bending and straightening;
• joint-line, hamstring and calf assessment;
• ligament stability and selected meniscal tests where appropriate;
• walking, squat, calf raise or other functional tasks when safe;
• pulses, skin colour, temperature, sensation and muscle power;
• hip and lower-back assessment when referred pain is possible.

No single manoeuvre confirms every cause. Vascular concern changes the pathway and may require urgent hospital or imaging assessment rather than routine musculoskeletal treatment.

Which scan is considered for pain behind the knee?

• Ultrasound: may identify a cyst or superficial tendon problem, assess a fluid collection and guide selected procedures. A dedicated venous ultrasound is used when DVT is suspected.
• X-ray: may help assess osteoarthritis, fracture, alignment or another bone-related question.
• MRI: shows menisci, ligaments, cartilage, bone marrow and the wider soft-tissue region. It may be considered for persistent or unclear symptoms when the result may alter treatment.
• Vascular imaging: is selected when an arterial or venous problem is suspected and follows a different clinical pathway.

For chronic knee pain, the American College of Radiology rates plain radiography as usually appropriate initial imaging. MRI without contrast may become appropriate in selected situations after the initial X-ray, including when radiographs are negative or show an effusion.^[7] This is not a rule that every posterior-knee presentation requires both scans.

Treatment depends on the underlying cause

A Baker’s cyst may improve when the underlying joint irritation is managed. Treatment can include activity modification, rehabilitation, medication after suitability review, or aspiration or injection in selected circumstances. Direct cyst surgery is not the routine first response.

Meniscal, osteoarthritis and tendon-related problems follow different pathways. For osteoarthritis, NICE identifies tailored therapeutic exercise and weight management where relevant as core treatments.^[4] Tendon or muscle recovery generally requires a staged return to load rather than repeated rest alone.

Passive modalities may help selected patients with symptom control, but the evidence, expected duration, risks and role within rehabilitation should be discussed. A DVT, arterial condition or infection requires medical management and should not be treated as an ordinary muscle problem.

Patient-specific approaches

If you can feel a lump behind the knee

Do not assume every lump is a Baker’s cyst. Note whether it is new, enlarging, tender, firm or pulsating and whether the calf is swollen. Examination and often ultrasound can help define the structure.

If the pain started after running

Review changes in distance, hills, pace and strength training. A local muscle or tendon problem may be considered, but joint swelling, meniscus and bone stress should remain in the differential if symptoms persist or affect walking.

If your calf suddenly swelled after a long flight

Seek urgent medical assessment for possible DVT. A known Baker’s cyst does not safely explain new one-sided calf swelling after travel, and routine knee treatment should wait until vascular concern is addressed.

If you need a quick return to work

Clarify whether work involves prolonged standing, stairs, driving, lifting or kneeling. Temporary duty changes may preserve function, but rapid symptom relief should not bypass assessment of a clot, infection, major injury or expanding mass.

If you have already tried exercises

Review whether the programme targeted the suspected joint, cyst-related, calf or hamstring contributor and how symptoms responded over the following day. Persistent swelling or an enlarging lump may justify a revised diagnosis and investigation rather than more of the same exercise.

If you prefer passive treatment or want the cyst drained

Discuss the goal and likelihood of recurrence. Aspiration or other symptom-relief measures may be considered for selected patients, but treating only the cyst may not address the joint problem producing excess fluid. The timing and degree of response cannot be promised.

If you want an MRI

MRI may add useful detail for meniscus, ligament, cartilage, bone marrow or an unclear mass, but ultrasound can be more direct for some cyst and vascular questions. Examination helps select the scan that matches the suspected cause.

Integrated doctor and physiotherapy management

The Pain Relief Clinic has in-house physiotherapists for integrated management with the licensed medical doctor. When physiotherapy is clinically appropriate, the team can align the working diagnosis, symptom and swelling management, movement assessment, progressive rehabilitation and decisions about imaging or other treatment. Physiotherapists practising in Singapore are regulated by the Allied Health Professions Council, which maintains a public register.^[8]

The clinic can assist with clinically appropriate referrals and supporting paperwork for insurance claims. Coverage, documentation requirements, pre-authorisation and reimbursement depend on the patient’s policy and insurer. A referral, form or medical report does not guarantee approval or payment.

Frequently asked questions

Is pain behind the knee usually a Baker’s cyst?

A Baker’s cyst is one common possibility, especially with fullness or a lump, but meniscus, joint swelling, tendons, muscles, nerves and veins can also cause posterior symptoms.

Can I tell a ruptured cyst from a blood clot?

Not reliably at home. Both may cause calf pain, swelling, warmth or colour change. New or rapidly worsening calf symptoms need urgent clinical assessment.

Should a Baker’s cyst always be drained?

No. Some improve when the underlying joint condition is managed, and some are asymptomatic. Aspiration may be considered in selected circumstances, with discussion of recurrence and procedural risks.

Can an X-ray show a Baker’s cyst?

An ordinary X-ray does not directly show the cyst well, but it can identify osteoarthritis or bone changes that may be related. Ultrasound or MRI may demonstrate the cyst when clinically indicated.

When should I arrange a non-urgent appointment?

Arrange assessment if posterior pain or a lump persists, repeatedly returns, limits walking or bending, interrupts sleep, or occurs with joint swelling, catching or reduced movement. Seek urgent care for calf swelling, circulation symptoms, severe illness or major trauma.

Key points

• Pain behind the knee may arise from the joint, a cyst, tendons, muscles, nerves or blood vessels.
• A Baker’s cyst often reflects an underlying knee condition rather than an isolated problem.
• A ruptured cyst and DVT can produce similar calf symptoms.
• New one-sided calf swelling requires timely medical assessment.
• Ultrasound, X-ray and MRI answer different clinical questions.
• Treatment should address the underlying cause and the patient’s functional priorities.

Arrange a posterior-knee-pain assessment in Singapore

If pain or a lump behind the knee persists, limits movement or occurs with joint swelling, a structured assessment can help identify the appropriate investigation and treatment pathway. Seek urgent care rather than a routine appointment for the warning features described above.

The Pain Relief Clinic
350 Orchard Road, #10-00 Shaw House, Singapore 238868
Call or WhatsApp: +65 9068 9605

Not every investigation or treatment discussed on this page is required, offered or recommended by the clinic. Availability and permissible use of tests, devices and treatments in Singapore may change with evolving Ministry of Health, Health Sciences Authority and Singapore Medical Council requirements. Individual decisions require clinical assessment and should follow current Singapore regulations and professional guidance.

References

1. Singapore Civil Defence Force. Emergency Medical Services.
2. American Academy of Orthopaedic Surgeons. Baker’s Cyst (Popliteal Cyst).
3. NHS. Baker’s Cyst.
4. National Institute for Health and Care Excellence. Osteoarthritis in over 16s: diagnosis and management (NG226).
5. American Academy of Orthopaedic Surgeons. Meniscus Tears.
6. National Institute for Health and Care Excellence. Venous thromboembolic diseases: diagnosis, management and thrombophilia testing (NG158).
7. American College of Radiology. ACR Appropriateness Criteria: Chronic Knee Pain.
8. Allied Health Professions Council Singapore. Registers of Allied Health Professionals.

Disclaimer: This information is provided for general reference only and should be used in consultation with your licensed medical doctor. It is not a diagnosis or a substitute for personalised medical advice. Seek urgent care for severe, rapidly worsening or warning-feature symptoms.

Reviewed by Dr Terence Tan — 12 September 2026